1. Have hack cases in mind before you lock. Preferably, prepare a input (stress test) generator.
2. After locking, go through the codes from bottom to top (in your room).
3. Look at the code quickly to guess whether it will fail your hack case. (If the code is too long, skip it and hope that it fail system test)
4. If the code looks suspicious, type it out and verify.

Only tip I have is that before you lock some problem, check your room to see which problems people got hacked on, statistically it should mean that those problems have weaker pretests, so you can have a higher chance on hacking those. On the other hand, when you see such problem you might want to double check you solution before locking it so no one will hack you on it.

"Writing code and then comparing with other? Did anyone tried automating it? Is it possible to build a tool that convert image to text and then stress testing it" — from a technical point of view it is surely possible, but it's not like you can't copy these codes directly for no reason. It is strictly prohibited to do so and doing that would be considered as cheating.